Thermal mass flowmeter
Mass flowmeter can be divided into two types:
A. Capillary thermal mass flowmeter
B- Thermal mass flowmeter in the path / inlet
- Incoming type
- Type in the path
Choosing the right design for the application depends on the following:
- Range of Dubai range
- Gas cleanliness
- Pipe and line dimensions
The two main types of thermal flow meters share the characteristics of flow rate and pipe dimensions.
Common CTMF mass flowmeter includes body and flow sensor. The temperature sensor is located inside the body of the flow meter. A certain part of the gas flow through the body of the flow meter passes through the working path and the gas flow is calculated by it.
A flow meter in which a common flow sensor includes a thin tube and two temperature sensors. Depending on the manufacturer, the heater can be combined with any of the temperature sensors or placed separately in the middle of the flow sensor, ie between the upstream temperature sensor T 1 and the downstream sensor T 2 .
A powerful power supply transmits constant heat to the flowmeter sensor. In the absence of motion in the fluid, both sensors measure the same temperature. When the flow rate increases, the heat is transferred from the upstream sensor T 1 and transferred to the downstream sensor T 2. The bridge circuit interprets this temperature difference and is generated by the output signal of the flow amplifier. The difference between the measured temperature between the sensors and the flow rate is mass.
The flow sensor measures the mass flow rate as a function of temperature difference.
Flow in thermal flow meters sensors is accurate only at low rates. Therefore, to measure higher rates, it is necessary to distribute the entire flow. This fluid distribution in the body is equipped with a linear flow element. This element produces a linear drop and pressure that is proportional to the mass flow.
The range and range of this type of flow meter is directly affected by the specific heat of the gas c p . Calibration is not possible for all process gases. If the gas is corrosive or hazardous, reference gases such as air and nitrogen are usually used for calibration.
Since a large amount of heat is transferred in high flows, in this equipment, practically only very low flow rates can be accurately measured. As a result, linear flow elements are commonly used for CTMF flowmeters. Which are located in the main line and cause a certain proportion of Dubai to cross in different areas of the crossing line. The end of the capillary tube is connected to the outlet and inlet of the flow linear manifold. This structure fixes the gas passage ratio through the capillary section. In this type of design, the heater and temperature sensors are placed in the capillary tube instead of the main path. Of course, there is another plan that the sensors are not located directly in the main path, ie without the capillary tube and the Dubai linear element.
Characteristic effects of gas:
The CTMF flowmeter should only be used to measure dry and clean gases. Impregnated vapors are condensed and therefore should be avoided to prevent the sensor from clogging and biting.
As can be deduced from the previous equations, the calibration of the CTMF flowmeter is dependent on the gas profile. The CTMF is therefore an inferential mass flow meter. Although the CTMF flowmeter responds to changes in mass flow rate, its calibration depends on process gas and operating conditions. Changes in fluid characteristics such as gas composition, pressure and process temperature that cause a change in the amount of specific heat lead to a change in the flowmeter efficiency. When this happens, the changes must be estimated. To determine the best flowmeter for the application, it is important to determine the range of conditions to which the equipment is exposed. These conditions include the following:
- Operation flow rate
- Process gas specifications
- Process pressure range
- Process temperature range
Temperature changes affect the behavior of the flowmeter. Estimates of these effects are generally included in the transmitter. Temperature changes may cause the equipment output to offset at zero flow. Therefore, the output of the equipment should be zero at the operating temperature of the process. The effects of temperature are generally determined as a percentage or degrees Celsius.
In addition, temperature changes may affect the amount of specific heat at which the flowmeter is calibrated.
Pressure changes may affect the specific gas heat or flow rate between the sensor and the flow linear element. Some manufacturers calibrate this equipment under specified pressure conditions to reduce it.
A pressure drop occurs when the fluid passes through the flowmeter. The continuation of this decrease is a function of the pressure difference in the two Dubai linear elements related to the equipment.
- Non-contact without input sensor
- No obstruction in the fluid path
- Less maintenance needs
- Suitable only for low flow gases
- It is prone to corrosion and wear.